Recent statutory limitations on the exhaust emissions of internal combustion engines have resulted in an intensive investigation of techniques for limiting exhaust emissions and development of the Otto engine, for the purpose of reducing exhaust emissions to satisfy statutory levels. Moreover, limitations on available fuel reserves have stimulated research to improve fuel ecomony and to study the feasibility of using alternative types of fuels for internal combustion engines. The combustion portion of the operational cycle of an internal combustion engine is the basic factor upon which economy and exhaust quality depend. Analysis of the combustion process serves to clarify the influence of various parameters with respect to engine efficiency and the creation of undesired exhaust components.
Albeit there are a number of ways to operate an internal combustion engine with a minimal amount of harmful exhaust components, not each of these solutions is a rational and scientific-like approach to the problem, especially when favorable fuel consumption is also a requirement. Of prime consideration is the investigation of those solutions which provide the most complete utilization of the fuel supplied to the engine cylinder, without afterburning the catalytic reactors, and a reduction of harmful exhaust pollutants. This requires an extensive analysis of the combustion process and a maximalization of all factors which influence exhaust emission and fuel consumption.
Of these factors, the one having the largest influence, the air factor .lambda., (the ratio of the actual fuel-air mixture to stochiometric fuel-air mixture) on combustion and exhaust emission has been investigated a number of times. For a range of .lambda. = 0.8 - 0.95 the shortage of oxygen produces incomplete combustion, which results in a relatively high carbon monoxide and high hydrocarbon concentration in the exhaust. The air shortage prevents the formation of large NO quantities even though the maximum combustion temperatures are quite high. An increase in the air-factor lowers the amount of carbon monoxide and hydrocarbon concentrations considerably. Minimal hydrocarbon quantities occur in a range of .lambda. = 1.1 - 1.2. This is the range of best ecomony. The high temperatures and sufficient air quantities desirable for oxidation of carbon monoxide and hydrocarbons cause a sharp increase in NO concentration, so that the maximum quantity of nitrogen monoxide takes place within the same air-factor range where the concentration of unburnt hydrocarbons is a minimum. As .lambda. exceeds 1.2, the concentration of nitrogen monoxide drops off sharply, so that from a standpoint of the decrease in harmful components in the exhaust, an air-factor range between 1.3 and 1.4 is desired.
Engine operation with lean fuel mixtures has further decided advantages. Leaner mixtures cause an increase in the thermal efficiency as a result of more favorable thermodynamic characteristics of the mixture. A deterioration in the quality and mechanical efficiency with a more lean mixture leads to a decrease in the effective efficiency. An optimum range of operation is obtained when the air-factor lies in a range between 1.2 and 1.4.
In order to reduce the exhaust emissions of the engine to as great a degree as possible and, at the same time, obtain a high engine efficiency, the following considerations must be taken into account.
(1) The operation cycle must terminate at moderate peak pressures and low peak temperatures, so that the quantity of nitrogen monoxide remains as low as possible.
(2) The temperatures during expansion should be relatively high in order to achieve after-oxidation of the non-burned hydrocarbons and carbon monoxide.
(3) In order to assure sufficient oxygen for a complete as possible combustion and after-oxidation of the hydrocarbons and carbon monoxide, the operational cycle must have an air-factor ratio .lambda. = 1.3 - 1.4.
(4) Differences in successive cycles should be as small as possible. A large non-uniformity of successive cycles reduces the desired and effective efficiency, especially in the air-excess area. Through a decrease in these fluctuations, the composition of exhaust gases from an Otto engine is considerably improved.
The operational cycle having low peak and relatively high expansion temperatures may be realized by delaying the combustion. This is possible by retarding the ignition point or through the use of a considerably lean fuel mixture. Both of these techniques impair indicating data, however. Particularly, the advantages of employing an Otto engine with a lean fuel mixture cannot be utilized since a satisfactory operation of this type of engine is only possible with a small air-factor, lying in a range of to about 1.3. With today's standard engines, there is a trend to employ leaner fuel mixtures with .lambda.&gt;1, resulting in a deterioration of the engine operation. By operating the engine with a even leaner mixture both ignition itself and flame-spread deteriorate. The number becomes even larger for those cycles for which the flame, during combustion and expansion, does not act upon the entire charge, so that combustion continues after the exhaust valve is opened. If the fuel mixture is too lean, the presumed combustion portion of an individual cycle does not occur as intended and ignition and combustion discontinue completely, so that stable operation of the engine is no longer possible. One of the results of a poorer combustion process is an increase in the non-uniformity of the combustion cycles, so that the mean pressure falls off and the economy of the engine operation deteriorates. At the same time, there is an increase in the concentration of unburned and partially burned hydrocarbons.
In view of the significance of exhaust emissions and fuel economy on present-day developments in internal combustion engines, extensive research has been conducted for the purpose of extending the operational range of an Otto engine, with respect to the air-excess area. Although there are various techniques to expand the air-factor ratio, for a conventional Otto type engine, improvements in the combustion process in the lean zone, nevertheless, are accompanied by an increase in the combustion temperature and, therewith, the amount of nitrogen monoxide emissions.
One possibility for improving the combustion of a lean fuel mixture is through the use of a so-called stratified charge. The basic premise behind the use of a stratified charge is the preparation of a relatively rich fuel mixture in the area of ignition adjacent to the sparkplug and the use of a very lean fuel mixture in the remaining portion of the combustion chamber which can then be burned quite well. By this technique, the operation of the engine produces a very good air-factor considerably larger than .lambda. = 1.0. Favorable conditions for ignition and flame propagation will result in an almost complete oxidation of the fuel and very little production of incomplete combustion components, namely carbon monoxide and hydrocarbons. By controlling the ignition and the starting phase of the combustion cycle, with an air-factor of .lambda. = 0.5 - 0.8 and, for the main combustion, an air-factor of .lambda. = 1.5 - 2.0, and air-factor of maximum NO formation occurring at about .lambda. = 1.1 is prevented.
Although there is a large amount of documentation on the application of the stratified charge approach to different types of engines, none finds a better use than the conventional Otto engine, particularly when considering its overall effect, from the standpoint of power output, economy, simple construction and operating reliability.
One possibility of employing charge stratification consists in the direct injection of fuel into the undivided combustion chamber, so that stratification is produced through a directed swirl-movement of the air. By this process, the mixture in the vacinity of the sparkplug is enriched and yet is still ignitable, by the high total air-factor. Of decisive importance in this type of system is injection pressure and the direction in which the fuel is injected, tha positional alignment of the sparkplug and the injection nozzle and, above all, the velocity of the flow of the air. Since the intensity of the air swirl is proportional to the engine speed of rotation, operational difficulties are encountered within a large speed and load range typically required for vehicle engines.
Charge stratification can also be obtained through the use of a divided combustion chamber, and with the aid of an auxiliary combustion chamber. In this case, a lean fuel mixture is drawn into the cylinder, whereas enrichment in the auxiliary combustion chamber is effected by means of an injection nozzle or an additional inlet valve. These arrangements are basically independent of the relative speed and load variations and, accordingly, are well suited for vehicle engines.
For a stratified-charge engine having a divided combustion chamber, it is possible to construct the auxiliary combustion chamber to be relatively small with about 3 to 15% of the compression volume or relatively large with about 20 to 60% of the compression volume. By dividing the combustion chamber into a main and an auxiliary combustion chamber, and by charge stratification, the degree of flamability of the lean fuel mixture is considerably improved. Ignition of the mixture for variable operating conditions, through these means along, however, cannot be guaranteed. The intensive turbulence of the charge in the auxiliary combustion chamber may severely impair the ignition and flamability requirements.
The combustion process in an Otto-type engine consists of the following phases:
Phase 1 -- Ignition and formation of a stable flame core (starting phase of the combustion). This phase is often designated as the ignition delay.
Phase 2 -- The main phase of combustion, during which the main quantity of fuel burns.
Phase 3 -- Afterburning.
The requirements for optimum rundown of the individual phases are variable and contradictory. The duration and performance of the first phase depend upon the ignition conditions, the mixture composition, the intensity of the turbulence, pressure and temperature. Critical conditions for the spark ignition of the fuel with layered combustion are then obtained, when a spherical volume with a radius R.sub.Kr is heated to a combustion temperature Tv. The minimum radius R.sub.Kr of the flame core, necessary for the ignition of the mixture, is: EQU R.sub.Kr .gtoreq. 2.sqroot.2 .times. .delta..sub.F1 .apprxeq. 3F1,
wherein small .delta..sub.F1 represents the thickness of the flame zone.
After ignition of the mixture within a volume having a critical radius R.sub.Kr has occurred, nothing prevents further spread of the flame. For R&lt;R.sub.Kr, the flame goes out since the heat yield at the unburned portion of the mixture is larger than the heat input through the combustion within the volume R. In the event of an intensively turbulent flow, as is customary with Otto engines, ignition and flamability requirements are considerabily more complicated. The possibility exists that the flame core, subsequent to ignition, first develops undisturbed and then ceases to burn. The critical ignition requirements can be satisfied initially and then disturbed, so that the flame develops slower than would be the case with layered flow.
For the ignition and development of a stable flame core, it is, accordingly, necessary that the heat which is freed by chemical reaction become larger at the small mixture portion, engaged by the spark, than the heat carried away from its surface. Ignition is to take place in the quiet center without the presence of a turbulent flow. High flow velocities also lead to a decrease in ionization and require and increase in the minimum ignition energy.
Combustion in the second phase differs from combustion in the first phase. After the formation of the stable flame core necessary for further runoff of the combustion reaction, an intensive turbulence is necessary for a quick and complete combustion. In conventional Otto engines, and in known stratified charge engines, separation of the individual phases of the combustion process and creation of optimum conditions are necessary for each phase.
An optimization for each individual combustion phase was sought through various stratified engines of the type described above having a main combustion chamber, an auxiliary combustion chamber and an ignition chamber. Mixture compressing external auto-ignited internal combustion engines of this kind, having charge stratification, are described in German Laid Open Publication Number 2448405, wherein the volume of the auxiliary combustion chamber and the volume of a third combustion chamber, referred to as a rest-gas chamber, satisfy the relationship: ##EQU1## where V.sub.z = the volume of the rest-gas chamber and V.sub.N = the volume of the auxiliary combustion chamber. It has been demonstrated, on the basis of a relatively large volume of the ignition chamber in relation to the volume of the auxiliary combustion chamber, that a considerable burnt rest-gas quantity remains, with a load change which strongly impairs the ignition capability of the subsequent charge in the ignition chamber. Also, charge turbulence is present which disadvantageously affects the ignition of the charge in the ignition chamber.